Ambient temperature X-ray diffraction data were collected at different pressures from two crystals of β-As 4 S 4, which were made by heating realgar under vacuum at 295 C for 24 h. These data were used to calculate the unit-cell parameters at pressures up to 6.86 GPa. Above 2.86 GPa, it was only possible to make an approximate measurement of the unit-cell parameters. As expected for a crystal structure that contains molecular units held together by weak van der Waals interactions, β-As 4 S 4 has an exceptionally high compressibility. The compressibility data were fitted to a third-order Birch–Murnaghan equation of state with a resulting volume V 0= 808.2 (2) Å 3, bulk modulus K 0= 10.9 (2) GPa and K′= 8.9 (3). These values are extremely close to those reported for the low-temperature polymorph of As 4 S 4, realgar, which contains the same As 4 S 4 cage-molecule. Structural analysis showed that the unit-cell contraction is due mainly to the reduction in intermolecular distances, which causes a substantial reduction in the unit-cell volume (~ 21% at 6.86 GPa). The cage-like As 4 S 4 molecules are only slightly affected. No phase transitions occur in the pressure range investigated.